This invention relates generally to a method for preparing lithium aluminum hydride directly from a slurry of sodium aluminum hydride in toluene.
Preparation of lithium aluminum hydride via the metathesis of sodium aluminum hydride and lithium chloride is well known. For example, U.S. Pat. No. 3,207,570 discloses a procedure for performing this reaction in tetrahydrofuran, and in mixtures of tetrahydrofuran with other ethereal solvents. In this procedure, sodium aluminum hydride is introduced as a solid or as a solution in tetrahydrofuran. However, sodium aluminum hydride typically is available commercially as a slurry in toluene. Prior to performing the metathesis reaction, the sodium aluminum hydride must be isolated from the slurry as a solid, with inevitable losses of material.
A method for converting the toluene slurry directly to a solution of lithium aluminum hydride, without prior removal of toluene and isolation of solid sodium aluminum hydride would be more efficient and would be commercially valuable.
The present invention is directed to a method for preparing a lithium aluminum hydride solution comprising lithium aluminum hydride, toluene and tetrahydrofuran by the steps of: (a) combining lithium chloride, tetrahydrofuran, and a slurry of sodium aluminum hydride in toluene; and (b) allowing the mixture formed in step (a) to react to form a product mixture comprising lithium aluminum hydride, sodium chloride, tetrahydrofuran and toluene.
Unless otherwise specified, all percentages herein are stated as weight percentages, temperatures are in xc2x0 C., and volumes in mL.
In one embodiment of the invention, a lithium aluminum hydride solution comprising lithium aluminum hydride, toluene and tetrahydrofuran is prepared by the steps of: (a) combining a solution of lithium chloride in tetrahydrofuran with a mixture comprising sodium aluminum hydride, toluene and from 25% to 98% tetrahydrofuran; and (b) allowing the mixture formed in step (a) to react to form a product mixture comprising lithium aluminum hydride, sodium chloride, tetrahydrofuran and toluene. The mixture comprising sodium aluminum hydride, toluene and from 25% to 98% tetrahydrofuran is produced by combining tetrahydrofuran with a slurry comprising sodium aluminum hydride and toluene. Preferably, the solution of lithium chloride in tetrahydrofuran is added to the mixture containing sodium aluminum hydride.
In another embodiment of the invention, a lithium aluminum hydride solution comprising lithium aluminum hydride, toluene and tetrahydrofuran is prepared by the steps of: (a) combining an amount of tetrahydrofuran with a slurry comprising sodium aluminum hydride and toluene; said amount being sufficient to form a mixture comprising sodium aluminum hydride, toluene and from 25% to 98% tetrahydrofuran; (b) combining the mixture formed in step (a) with a mixture of lithium chloride and tetrahydrofuran; and (c) allowing the mixture formed in step (b) to react to form a product mixture comprising lithium aluminum hydride, sodium chloride, tetrahydrofuran and toluene. The mixture of lithium chloride and tetrahydrofuran contains solid lithium chloride.
In the method of this invention, a slurry comprising sodium aluminum hydride (xe2x80x9cSAHxe2x80x9d) and toluene preferably has from 1% to 75% SAH and from 25% to 99% toluene, more preferably from 10% to 60% SAH and from 40% to 90% toluene, and most preferably from 30% to 50% SAH and from 50% to 70% toluene. Tetrahydrofuran (xe2x80x9cTHFxe2x80x9d) is added to the slurry to produce a mixture comprising SAH, toluene and tetrahydrofuran. THF is added in an amount sufficient to produce a mixture that contains from 2% to 75% toluene and from 25% to 98% THF, more preferably from 25% to 50% toluene and from 50% to 75% THF, and most preferably from 30% to 40% toluene and from 60% to 70% THF.
In one embodiment of the invention, the amount of THF is sufficient to produce a solution comprising SAH, toluene and tetrahydrofuran. In this embodiment, preferably the mixture is agitated for a sufficient period of time to allow formation of the solution.
In one embodiment of this invention, lithium chloride is added as a mixture with THF; preferably, the percentage of lithium chloride in the lithium chloride-THF mixture is from 1% to 50%, more preferably from 2% to 35%, and most preferably from 3% to 6%. The lithium chloride-THF mixture is added to the mixture comprising SAH, toluene and THF, and the resulting mixture comprising SAH, toluene, THF and lithium chloride is allowed to react to form the product mixture. It is preferred that the lithium chloride-THF mixture is substantially free of any solvent other than THF, and that the SAH mixture is substantially free of any solvent other than THF and toluene. In a preferred embodiment of this invention, the lithium chloride-THF mixture is a solution.
In another embodiment of the invention no THF is added with the lithium chloride, i.e., the lithium chloride is added as a solid to the mixture comprising SAH, toluene and THF.
The molar ratio of SAH to lithium chloride prior to reaction preferably is from 1:10 to 10:1, more preferably from 1:5 to 1:0.75, and most preferably from 1:1.1 to 1:0.95. Preferably, the mixture is allowed to react to form the product mixture by heating to a temperature from 22xc2x0 C. to 125xc2x0 C., more preferably from 50xc2x0 C. to 80xc2x0 C., and most preferably from 60xc2x0 C. to 70xc2x0 C.
In a preferred embodiment of the invention, the product mixture contains lithium aluminum hydride, THF, toluene and sodium chloride. Preferably, the product mixture is filtered to remove sodium chloride. When the SAH mixture and the lithium chloride are substantially free of solvents other than THF and toluene, the resulting product solution is a solution of lithium aluminum hydride in THF/toluene. Optionally, THF is removed from the solution to produce a solution having a higher concentration of lithium aluminum hydride. Typically, removal of THF is accomplished by distillation.